Central Tendency
Introduces the mean, median, and mode, and summarizes when each measure is most appropriate for describing the typical value of data.

Dispersion
Covers methods of measuring variation in data, including range, average deviation, variance and standard deviation.

Standard Deviation and Variance
Defines standard deviation and variance. Explains their advantages over other measures of dispersion and how to calculate them for both populations and samples.

Probability
Discusses concepts of probability, outcomes and events, the law of addition, conditional probability, and the law of multiplication.

Permutations and Combinations
Counting methods for determining the number of ways that members of a group can be arranged when order is a distinguishing factor (permutations) and when it is not (combinations).

StatSoft Electronic Statistics Textbook
Statistics reference covering a wide range of concepts and applications, including basic concepts, ANOVA, cluster analysis, data mining techniques, discriminant analysis, experimental design, factor analysis, linear regression, multidimensional scaling, quality control charts, and more.

Recommended Reading

Gonick, Larry, Smith, Woollcott, The Cartoon Guide to Statistics